import React, {useEffect, useState} from 'react';


export default function App(){
  let[opacity,setOpacity]=useState(1)
  // 初始化的时候添加一个定时器
  useEffect(()=>{
   const id= setInterval(()=>{
      opacity-=0.1;
      if(opacity<=0){
        opacity=1
      }
      setOpacity(opacity)
    },100);
    return ()=>{
      clearInterval(id)
    }
  },[])


  return (
    <div>
    <h1 style={{opacity}}>reacthook  一般一般</h1>
    </div>
  )
}